1. Field of the Invention
The present invention relates to an active matrix display device incorporating a device circuit, and in particular relates to an active matrix display device in which the power supply wiring is improved.
2. Description of the Related Art
Of display devices, liquid crystal display devices have the characteristic advantages that they are of small thickness and light weight, and can be driven by low voltage and color display can easily be obtained. In recent years they have come to be widely employed as display devices for personal computers or word processors etc. In particular, an active matrix type liquid crystal display device, in which thin-film transistors(TFTs) are provided as pixel switching elements at each pixel is an ideal system for display devices for OA use or full-color televisions. The reason is that, even for a large number of pixels, there is no deterioration of contrast or response and intermediate levels of luminance can be displayed.
Such an active matrix liquid crystal display device principally consists of two flat glass substrates(array substrate and counter substrate) and a liquid crystal layer sandwiched between these substrates. A color filter and transparent electrode(counter electrode)are formed corresponding to each pixel at the surface of the counter substrate, which constitutes one of the glass substrates. On the other substrate i.e. the array substrate, there are provided pixel electrodes consisting of transparent electrodes arranged in matrix fashion and TFTs whose source electrodes are connected to each pixel electrode. The gate electrodes of these TFTs are connected to address lines arranged in the X direction, while their drain electrodes are connected to data lines arranged in a direction at right angles to the address lines.
In a conventional active matrix liquid crystal display device constructed in this way, voltages corresponding to picture data can be selectively applied to each pixel electrode by applying respective address signals and data signals to the address lines and data lines with prescribed timing. The optical transmittance of the liquid crystal layer can be controlled by altering the voltage difference between the counter electrode and pixel electrode and any desired display images can be realized by this technique. Details are given in an article by T. P. Brody et al.(IEEE Trans. on Electron. Devices, VOL.ED. Nov. 20, 1973, pp. 995-1001).
Amorphous Si or polycrystalline Si etc. are typically employed as the semiconductor material forming the channel regions for the TFTs. In active matrix liquid crystal display devices, which have become most common in recent years, the TFTs for pixel switches are formed by amorphous Si on a glass substrate and a packaged LSI which is stuck on to the peripheral region of a glass substrate and connected to the TFTs which are used for pixel switching. An active matrix liquid crystal display device using polycrystalline Si achieved a further improvement of the active matrix liquid crystal display device.
Owing to the fact that the TFTs for pixel switching and the device circuit for applying drive signals to the address lines and data lines can be formed on the same glass substrate, such a liquid crystal display device has the advantages that the entire liquid crystal display device can be made of small size and high reliability of the wiring connections can be obtained. FIG. 1 is a view showing the construction of an active matrix liquid crystal display device using polycrystalline Si TFTs incorporating conventional device circuits. An array substrate 1 is formed with an array of TFTs 11, data line drive circuits 2 and address line drive circuit 3 etc. In addition, this array substrate 1 and a counter substrate 6, formed with counter electrodes 13, are arranged facing each other, and a liquid crystal layer 12 is sealed between these substrates 1 and 6 to complete the construction. Drive circuits 2,3 input prescribed signals from respective signal input terminals 9,10 and apply drive signals to respective data line 4a,4b,4c.about. and address lines 5a,5b,5c.about., thereby driving the TFTs 11 of each pixel.
The problem with an active matrix liquid crystal display device incorporating such a drive circuit was how to make the low impedance of the power wirings 7,8 that apply power supply voltage to drive circuit 2,3. Usually, special materials such as Ta, TaMo, or MoW that are not used in LSIs etc. are used to provide wiring material of low resistance and sufficient ability to withstand the highly acidic etching liquids such as aqua regia that are employed in the etching step of the transparent electrodes such as ITO in an active matrix liquid display device. No better materials have been discovered and resolution of the problem of the wiring material is difficult. In order to obtain lower resistance, it was therefore necessary to adopt expedients such as increasing the wiring width or providing additional power source supply terminals 7b,8b,7c,8c at a plurality of locations on substrate 1 rather than just power source supply terminals 7a,8a from the outside.
However, increasing the width of the power source wiring tends to increase the area dedicated to utilities in other words the border region or non-display region at the periphery of the display region where the pixels are formed, and thus tends to increase the size of the substrate and hence the liquid crystal display device as a whole.
Also, with a construction in which power supply terminals are provided in a plurality of locations, connections with external wiring are increased, and the number of places where different sets of wiring cross each other, such as for example, the places where the lines to signal input terminals 9 and address line drive circuit 3 are increased, giving rise to the risk of short circuiting. In order to eliminate such points of cross-over wiring detours are necessary, but there increase the size of the border region constitution the wiring region, so in this respect also miniaturization of the liquid crystal display device cannot be achieved.
Summarizing these problems, miniaturization of liquid crystal display device as a whole cannot be achieved because the non-display region at the periphery of the display region cannot be reduced in size. The arrangement is not particularly desirable from the point of view of reliability because the proliferation of points of cross-over of power source wiring and other wiring increases the risk of short circuiting between wirings.
Such problems do not manifest themselves with severity in small liquid crystal display devices of about 5-inch size using polycrystalline TFTs, but silicon cause as particularly important problems in connection with large active matrix liquid crystal display devices of 10-inch size or more.